Selecting and achieving the proper air barrier

May 1, 2012

All photos courtesy Sto Corp.[1]
All photos courtesy Sto Corp.

By John Edgar
Air barriers have been a requirement of the National Building Code of Canada (NBC) for many years, but not all design professionals fully understand what is involved in specifying one. An air barrier may be a material of many functions and the choice of one over another should reflect the needs of the particular project. Historically, the requirements for airtightness have been found under NBC Part 5, “Environmental Separation.” Requirements are now being refined with the introduction of the new National Energy Code of Canada for Buildings (NECB). Air leakage, moisture management, energy conservation, and fire safety are all factors that come into play.

Based on their location in a wall assembly, many air barrier materials are also assumed to function as a water-resistive barrier (WRB). With this dual function, air barriers and WRBs must not only resist air leakage, but also prevent water penetration. In doing so, they need to remain durable and leak-proof under the stresses that may be experienced in service. A material may be airtight or leak-proof when subjected to a single test, but how will it perform when it is filled with holes from mechanical fasteners? Another important factor to consider is vapour permeability. As the insulation requirements increase, with more positioned outbound of the air barrier, vapour permeability requirements of the air barrier must be determined.

Various air barrier products—fluid-applied, polymer-based sheet wraps, and asphaltic self-adhered membranes—are being used in non-combustible construction, often without consideration of their potential impact on fire performance of a wall assembly. Selecting a material that meets all the requirements defined in Part 5 and the NECB may have unintended consequences with respect to Part 3. The best way to know if a product meets the requirements is to review the testing and code evaluation reports.

Air barriers and WRBs 
The Canadian Construction Materials Centre (CCMC) has developed a series of technical guides to evaluate materials not specifically identified in NBC. These guides outline testing requirements to demonstrate durability and compliance with the intent of the code. The development of the Technical Guide for EIFS is an example of the use of research-based test methods unique to exterior insulation finish systems.

An alternative approach would be to show equivalent or superior performance to materials that are acceptable and listed in the code. This approach can present challenges for CCMC where a product’s code acceptance is based on historical precedence and not on performance—one such example can be found with building felts.

While building felts are an acceptable material for a WRB, the ability to resist water penetration with mechanical fasteners does not currently require evaluation. This missing data makes evaluation of a new material against the performance of an existing code-accepted one difficult (and perhaps impossible) without comparing the two.

The introduction of new materials with a different approach to a common problem raises a new series of complications in evaluations. A good example is the development of the Technical Guide for EIFS with a liquid-applied (LA) water-resistive barrier. How should one evaluate a WRB that becomes part of the substrate material versus a loose-sheet material that is mechanically fastened?

For example, EIFS have unique qualities that include the ability to bond to the LA-WRB and resist the live loads imposed on the wall assembly. However, what happens to the membrane if nails ‘pop?’ What happens to sheathing joints when the wall racks and moves? What happens if the WRB is left exposed for a certain period? What happens when water makes its way back to the membrane and must be drained to the exterior?

In the case of EIFS with LA-WRBs, the EIFS industry and CCMC spent five years of research developing test methods that demonstrate durability of such water-resistive barriers. These methods were subsequently translated into Underwriters Laboratories of Canada (CAN/ULC) S716.1, Standard for Exterior Insulation Finish Systems–Materials and Systems. The collaboration between the EIFS industry and CCMC has resulted in material evaluations unparalleled in demonstrating durability and performance.

Sun Peaks Resort (pictured on page 18) is northeast of Kamloops, B.C. It includes an exterior insulation finish system (EIFS).[2]
Sun Peaks Resort (pictured on page 18) is northeast of Kamloops, B.C. It includes an exterior insulation finish system (EIFS).

Today, various manufacturers recognize the advantages of liquid-applied membranes and have introduced products that act as standalone air barriers and moisture barriers, independent of a specific cladding. CCMC has worked with numerous manufacturers to evaluate these materials as air barriers, air barrier systems, moisture barriers, or a combination of these functions.1

Most recently, a fluid-applied moisture barrier used inbound of insulation with any cladding was evaluated. The research to develop the technical guide involved testing water penetration resistance through the membrane at mechanical fastener locations and comparing performance with materials historically accepted by NBC. The requirements included durability testing from the Technical Guide for EIFS and from International Code Council (ICC) Acceptance Criteria (AC) 212, Water-resistive Coatings Used as Water-resistive Barriers Over Exterior Sheathing. These are extremely robust requirements that go far beyond testing a single material for one or two properties.

For a specifier or a building official, this research and development with CCMC provides a process for selecting a material or system that performs in the Canadian climate. A CCMC evaluation report provides an opinion evaluated materials meet NBC intent and requirements.

Fire testing
Building codes evolve to reflect the needs and experience of the times. Today, building construction integrates very sophisticated materials into wall assemblies that are measured in millimetres. These assemblies are designed to perform more functions than walls that used to be metres thick. New codes provide basic requirements relating to health and safety, while maintaining a degree of flexibility in meeting these objectives.

Without going into a detailed analysis of codes, it is clear walls must be airtight, watertight, thermally efficient, and fire-resistive. Buildings are now defined as being of “combustible” or “non-combustible” construction. Housing and small structures can be the former—typically wood frame. Institutional and many commercial projects are required to be of non-combustible construction, which often means steel frame, concrete, or masonry, based on building height and occupancy. Combustible materials may be used in non-combustible construction provided certain testing is performed and materials meet criteria established by the codes.

Examining two fire tests used to evaluate exterior wall materials and assemblies for use in non-combustible construction provides a better understanding of how new combustible materials may affect fire test performance. The two tests are:

This is the basic test that determines whether a material is ‘combustible.’ If a material is deemed to be combustible, then limitations for its use are outlined in NBC Part 3, “Fire Protection, Occupant Safety, and Accessibility.”

As thermal efficiency requirements are increased, foam plastic insulation has become an effective material in meeting the code’s objectives.

Foam plastic insulation is typically rated as combustible by this standard so it requires subsequent testing if used in a wall assembly. There are exceptions for other ‘minor combustible elements’ such as paint, air barrier ‘connective’ materials, sealants, wood trim, and other minor components, but the air barrier system is not exempt. NBC Article outlines the requirement for testing in conformance to CAN/ULC S134 for assemblies with combustible materials in non-combustible construction. This is the fire test developed when EIFS were introduced.

EIFS can also be specified on existing buildings, as in the case of this hotel-turned-condo.[3]
EIFS can also be specified on existing buildings, as in the case of this hotel-turned-condo.

CAN/ULC S134-92
This is a large-scale fire test for wall cladding assemblies. It was developed at the National Research Council-Institute for Research in Construction (NRC-IRC) National Fire Laboratory (NFL) as part of a research project to evaluate the best method for evaluating wall cladding performance when subjected to a fire source. EIFS assemblies were early subjects of research. Of the many methods assessed, this test best simulated actual fire performance. It is difficult to successfully simulate a real fire with a small-scale test.

The test apparatus is 10 m (33 ft) tall with an opening representing a window and a room behind the opening where the fire is generated. This simulates fire consuming all the combustible materials in a room, resulting in flames that burst out the ‘window’ and expose the wall surface. Propane is the fuel for the fire test, which runs for 25 minutes. The test measures the flame spread within the cladding assembly, how far it spreads up the exterior wall surface, and how much energy is contributed by the cladding.

For those who have witnessed the procedure, it is a hair-raising and singeing experience to see a roiling ball of plasma suddenly ‘flash over’ and billow out the window and up the wall. Casual observation is not an option for this test. Even standing on the other side of the laboratory, those watching are compelled to step back from the sudden flux of radiant energy as the fire bursts from the opening. (One develops a huge respect for firefighters who face this on a regular basis.)

Where do air barriers fit in this test? Foam insulations used in wall assemblies such as
EIFS and in other cladding cavities have been tested to demonstrate compliance with the code requirements. The question for the specifier is whether these assembly tests include an air barrier. This has become an issue in the United States where a similar test is run—National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components.

Recent concerns have been raised that some previously tested assemblies do not meet the requirements if an air barrier membrane is added. What had been treated as a minor combustible element appears to have a significant detrimental influence on performance. Now that air barriers and continuous insulation are both new requirements in the United States under the International Building Code (IBC), many companies are fire-testing assemblies to demonstrate compliance.

A specifier should obtain an evaluation that the air and moisture barriers used in a wall assembly have met the requirements of NBC Part 3.

Joint Base Lewis-McChord (Washington) has a new 10,668-m2 (35,000-sf) building—the SOF Aviation Battalion Education Center. It boasts a liquid-applied water-resistive barrier (LA-WRB).[4]
Joint Base Lewis-McChord (Washington) has a new 10,668-m2 (35,000-sf) building—the SOF Aviation Battalion Education Center. It boasts a liquid-applied water-resistive barrier (LA-WRB).

Liquid-applied water-resistive barriers
The evaluation of LA-WRBs has taken durability requirements beyond normal material testing.2 Evaluators asked the question, “What happens at joints?” In the United States, the ICC Evaluation Service (ES) determined racking a full-scale mockup before water penetration testing would be required. In Canada, initial research showed racking resistance was doubled with a LA-WRB. CCMC decided racking would be reduced so this type of stress would not be a durability factor.

The alternate test chosen was to bend the joint outward, the purpose being to stress the membrane over the joint to failure. Samples that survived were placed in a vice-like device and the joints were stretched 40 per cent. While stretched, the samples were subjected to environmental cycling; the joints were then tested for water leakage using a modified version of ASTM E 96, Standard Test Methods for Water Vapour Transmission of Materials, with 25 mm (1 in.)
of water above and desiccant below.

Moisture gain in the sheathing was measured. In the initial research conducted by Forintek Canada Corp., in Québec City, researchers noted samples left in the test apparatus for 82 days did not exhibit mould growth.

The CCMC Technical Guide for EIFS and, ultimately, ULC S716.1, require LA-WRB testing for:

The new CCMC Technical Guide for LA-WRBs requires testing based on both the ICC-ES Acceptance Criteria and the CCMC Technical Guide for EIFS, Appendix 4. Additionally, fastener penetrations are tested for water penetration under a range of pressure differences after heat aging. The LA-WRB performed six times better than the standard code-recognized material—building felt.

LA-WRBs have been tested to ASTM E 2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies, and meet the requirements for an air barrier system. The system test includes joints, secondary connections, fasteners, and penetrations.

In addition to durability testing, LA-WRBs have been fire-tested as standalone components and as components in assemblies. In the United States, materials are required to be tested to ASTM E 84, Standard Test Method for Surface-burning Characteristics of Building Materials (similar to Canada’ s ULC S102, Standard Method of Test for Surface-burning Characteristics of Building Materials and Assemblies), to obtain a flame spread rating. Typical results are less than 25, which ranks a material in the Class A category. In Canada and the United States, EIFS assemblies with expanded polystyrene (EPS) insulation that is 100, 140, and 300 mm (4, 5.5, and 12 in.) thick have been successfully tested with a LA-WRB.

Air barrier use in building and construction is an important part of sustainable design, and is a required component in many green building rating systems and codes. These include:

performance Green Buildings; and

Air barriers have come into Canadian and U.S. building codes following different paths.

Originally, NBC introduced the requirements for air barriers to minimize condensation and bulk water entry into the building envelope. The importance of this was confirmed with Canada’s implementation of objective-based codes and recognition of the airtightness contribution to health and safety of the occupants. In the United States, air barriers have become a requirement of the International Energy Conservation Code (IECC) in recognition of a National Institute of Standards and Technology (NIST)3 study that calculated 40 per cent of energy consumption in cold climate buildings is lost due to air leakage.

Both approaches are valid and are converging. In December 2011, NECB was published with the energy conversation requirements for air barriers. Requirements for air barriers as protection against water condensation and penetration are currently being discussed for inclusion in IBC in the United States. The value of air barriers is now well recognized. The issues that remain are regarding air barrier materials now being promoted to comply with all aspects of the code, whether they are durable, and if they meet the specific requirements for the project.

1 An air barrier ‘material’ is a single component that has airtight properties. An ‘assembly’ is a series of materials integrated to become a single airtight component (e.g. a window). The complete air barrier around a building is the air barrier system.
2 An example of ‘normal’ material testing is ASTM E 96 water vapour transmission, which tests for a single characteristic of a material and infers performance. With the LA-WRB, extensive testing for durability under environmental stresses was required. Typical sheet ‘sheathing membranes’ will not pass the testing LA-WRBs go through.
3 For more, see “NIST Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use,” by Steven J. Emmerich, Timothy P. McDowell, and Wagdy Anis (June 2005).

John Edgar is Sto Corp.’s technical director for Canada. He has worked for the company for more than 20 years, and is actively involved with the Air Barrier Association of America (ABAA), EIFS Council of Canada, Underwriters Laboratories of Canada (ULC), and numerous research organizations evaluating exterior insulation finish systems (EIFS) and liquid-applied  water-resistive barriers (LA-WRBs). Edgar has also served 13 years on the Environmental Separation Subcommittee for the National Building Code of Canada (NBC). He can be contacted via e-mail at

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